ANS is committed to advancing, fostering, and promoting the development and application of nuclear sciences and technologies to benefit society.
Explore the many uses for nuclear science and its impact on energy, the environment, healthcare, food, and more.
Division Spotlight
Reactor Physics
The division's objectives are to promote the advancement of knowledge and understanding of the fundamental physical phenomena characterizing nuclear reactors and other nuclear systems. The division encourages research and disseminates information through meetings and publications. Areas of technical interest include nuclear data, particle interactions and transport, reactor and nuclear systems analysis, methods, design, validation and operating experience and standards. The Wigner Award heads the awards program.
Meeting Spotlight
ANS Student Conference 2025
April 3–5, 2025
Albuquerque, NM|The University of New Mexico
Standards Program
The Standards Committee is responsible for the development and maintenance of voluntary consensus standards that address the design, analysis, and operation of components, systems, and facilities related to the application of nuclear science and technology. Find out What’s New, check out the Standards Store, or Get Involved today!
Latest Magazine Issues
Apr 2025
Jan 2025
Latest Journal Issues
Nuclear Science and Engineering
May 2025
Nuclear Technology
April 2025
Fusion Science and Technology
Latest News
Norway’s Halden reactor takes first step toward decommissioning
The government of Norway has granted the transfer of the Halden research reactor from the Institute for Energy Technology (IFE) to the state agency Norwegian Nuclear Decommissioning (NND). The 25-MWt Halden boiling water reactor operated from 1958 to 2018 and was used in the research of nuclear fuel, reactor internals, plant procedures and monitoring, and human factors.
Coleridge A. Wilkins
Nuclear Science and Engineering | Volume 14 | Number 4 | December 1962 | Pages 346-357
Technical Paper | doi.org/10.13182/NSE62-A26241
Articles are hosted by Taylor and Francis Online.
This paper involves the application of some modern probability methods to the problem of the slowing-down of neutrons, and attempts to provide a framework both for the exposition and computation of various phenomena in this field. It is shown that under certain conditions, the slowing-down of neutrons is a renewal process and renewal theoretical results apply. For instance, if scattering is isotropic and all cross sections similarly varying, then (in lethargy) slowing-down is certainly a renewal process. After investigating some aspects of moderation in hydrogen, and some incidental extensions of results for hydrogen, it is shown that for renewal types of slowing-down from a monoenergetic source, the Laplace transform of the rth moment of the number of collisions at lethargies below u may be obtained by differentiating (with respect to t) a generating function of the form This form applies to some types of anisotropic scattering as well as to isotropic scattering. The expressions derived are then extended to a distributed source by a method from reactor theory, and some resulting expressions are checked against corresponding renewal formulas. The asymptotic distribution of the number of collisions to slow down is found from renewal theory, from which it is shown that when scattering is isotropic, the spread of the number of collisions to thermalize in a light moderator is relatively greater than in a heavy moderator, the coefficient of variation being proportional to for a light species and for a heavy one. The asymptotic form of the density of nth collisions is investigated, and the form derived is compared with another ascribed to Dancoff. The preceding theory is then applied to a particular case of anisotropic scattering which occurs above about 100 kev. Finally, an exact expression is obtained (for similarly varying cross sections and zero absorption) for the probability that, in a mixture of n species, a neutron has r collisions at lethargies below u, precisely k of which are with a given species. The given species is then taken to be very heavy and the exact expression approximated accordingly.
